1. Field of the Invention
The present invention relates to electronic braking systems (EBS) for road vehicles and in particular to said systems intended for vehicles which operate with a trailer.
2. Discussion of the Background
In a towing vehicle and trailer combination, it is conventional practice for the braking requirements of the trailer to be signalled from the driver demand (brake pedal depression) in the towing vehicle/unit via the EBS equipment thereon, by either coded electrical or analogue pneumatic signals.
In the ideal case, this braking signal should be set and controlled in such a way as to achieve a balance in braking between the towing and trailer vehicles so that each contributes fairly to the overall retardation, giving clear improvement in overall brake lining wear on the combination and in the stability of the vehicles during heavy braking.
Because towing vehicles are likely to tow many different trailers having unavoidably different braking characteristics, no fixed braking ratio between the towing vehicle and an assumed laden trailer will be satisfactory in maintaining the sought after balance. Proposals have been made already for a learning or adaptive control system to be installed on the towing vehicle in order to assess the braking condition of the trailer and thereafter modify the braking command signals passed to the trailer on each driver braking demand in such a direction as to achieve the desired balance. Such systems are described in European patents 0370 671, 0385 648 and 0531 077 and have the common requirement of a means of measuring the forces exerted by the trailer, during braking, on the towing-vehicle-mounted coupling. Since such direct measurements require expensive sensors and circuitry if they are to be sufficiently rugged for duty on a range of heavy vehicles, there would be a real benefit in value engineering terms, if a means of indirect but practical assessment of the coupling forces could be provided from alternative lower cost sensors which in such a system could have other control benefits when installed on the towing vehicle and could thereby promote the acceptance of trailer control within the haulage industry and thus make benefits of economy and safety available to the truck operator.
There is known from EP-A- 0374 484 a tractor trailer brake control system where measurements are made of axle load on the tractor and which are used in determining the pressure level provided for the trailer so as to seek to achieve a balanced braking performance. The system disclosed in EP-A- 0374 484 has a trailer braking control system which is regulated so as to achieve balanced braking between the towing vehicle and the trailer utilizing the forces in the suspension components of two axles, the changes occurring therein during braking being reflected in these values.